Segregation defect of ZL205A alloy casting during practical production process was researched. The microstructure of the segregation, the compositions and the microhardness of the segregation phase, and the effect of segregation defect on the mechanical properties of the casting were studied. The results reveal that the chemical compositions of the segregation phase are Al3Ti, Al3V and Al3Zr, and the microstructure is in the form of rectangle or petal shape. The segregation defect has an obvious effect on the tensile strength and the elongation. But the microhardness of the segregation phase is higher than that of the matrix. Hot treatment can not eliminate the segregation. Actually the massive segregation belongs to the microsegregation. It is caused by downward movement of Al3Ti, Al3V and Al3Zr of heterogeneous nucleation particle to agglomerate during solidification. And the coarse massive structures in the ingot casting have certain morphological heredity in its formation.